Explain why pi-pi* transitions are the most useful transition in UV-Vis spectroscopy?
There are several possible electronic transitions in molecules. According to the molecular orbital theory, the energy levels of a molecule are located in such order: σ, π, n, π*, σ*. The most common transitions are σ-σ*, π-π*, n-π*. The σ-σ* transitions have the biggest energy difference which means that this the absorbed photon does not fall in the UV-Vis range (the wavelength is too small). The π-π* and n-π* transitions both belong to the UV-Vis range. The π-π* transitions are more useful than the n-π* transitions because the n-π* transitions correspond to the nonbonding electrons in the molecule, whereas the π-π* transitions correspond to the electrons of the conjugate system.